Key points of this article.
-RF antennas come in many forms, from flat antennas integrated into the chip to copper antennas printed directly on the PCB.
-When creating a layout with one or more antennas, it is important to ensure that the different circuit blocks of the PCB are isolated from each other.
-When designing an RF antenna, CAD tools should be used, which can help design isolated structures, transition structures, and even printed antennas for the PCB.
Today, it is hard to imagine a consumer electronics product that does not include an antenna, and even garage door openers can be connected to a cell phone via Bluetooth or WiFi. Every time a new RF antenna is added to a PCB layout, it creates new challenges for the RF designer, especially since current designs are again focusing on analog design skills. With so many RF features being added to new PCBs, how can designers ensure that the signal in the system is not corrupted and that signal integrity is maintained?
Some simple design choices can ensure that RF signals are not weakened by nearby digital components, but will also help prevent interference between multiple analog signals. While there are many RF design aspects to consider when designing a mixed-signal or full RF system, antenna design and layout are probably the two most important. Below we learn about RF antenna design in PCB layout and how to ensure analog signal integrity.
RF Antenna Design Basics
When designing a custom antenna or selecting a COTS antenna for an RF PCB, there are several basic points to follow. All RF antennas have some special characteristics that should be considered during the design phase. Each antenna needs to have the following components.
-Floating conductive radiator: the antenna unit used to emit radiation.
-Reference plane: the reference plane or unit of the antenna helps to determine the directionality of the antenna structure in each antenna mode.
-Feedline: feedline is used to transport the input signal from the RF element to the radiating antenna unit.
-Impedance matching network: The antenna typically has an impedance of about 10 ohms and therefore needs to be matched to the feedline impedance to prevent reflections and to ensure maximum power transfer at the desired carrier frequency and bandwidth.
Many standard antenna designs have been well studied. Many reference designs can be found on the Internet, which can then be copied into your own PCB layout. We can also find many design formulas for standard antenna structures in microwave engineering textbooks. Finally, if one wants to use a COTS RF antenna, there are many inexpensive designs available on the market. Regardless of which RF antenna you choose to use, it needs to be carefully placed in the layout to prevent interference between the various parts of the board.
RF antenna layout tips
After designing the antenna, you need to determine where on the PCB you should place it. RF designers can get some tips from mixed-signal designers (most RF boards are actually mixed-signal boards) to prevent interference between the RF front-end, back-end and digital sections.
-Efficient radiation: designed to ensure that the radiation from the antenna unit leaves the board and is not picked up by other structures in the PCB layout.
-Isolation: Again, we do not want multiple parts of the PCB layout to interfere directly with each other.
-Electromagnetic Compatibility (EMC): Finally, it is necessary to ensure that the layout does not receive signals from other devices that may emit signals in a wide range of frequencies.
In the actual design of a PCB, most design goals are competing, but there are two key points to follow that will help balance these design goals.
Keep circuit blocks separate from each other in the PCB layout
This is a basic mixed-signal PCB design point, and it also applies to the layout of the RF antenna. The antenna section needs to be placed on the board and separated from the other circuit blocks. In general, it is best to place the antenna section close to the edge of the board and away from other analog components. This limits the strong radiation to one location on the board and ensures minimal interference between the sections.
The challenge with meshing is to ensure that the return paths of the different sections do not interfere with each other, which would otherwise lead to noise coupling and crosstalk. Using the field solver integrated in the advanced PCB design tool will help to detect deviations in the return path when creating the layout. For high frequency designs, use a continuous ground plane structure to ensure a consistent return path.
Isolated antenna sections
Modern cellular phones and high-speed data networking devices employ creative isolation structures that have become the gold standard for RF isolation technology. Quite simply, isolation is the placement of some shielding around the RF sensitive components on the board to stop the propagation of waves between the transmitter and receiver. The table below describes some of the structures that can be used in the RF antenna section to isolate components, feed lines and antennas, or to isolate external noise sources.
Isolation structures are typically placed between RF components to prevent noise coupling and power exchange between them. Determining which isolation structure to use to ensure the integrity of the RF antenna signal is a complex design problem that has been thoroughly studied by the industry. If we are not experts in elliptic integration, we need to rely on electromagnetic (EM) field solvers to determine how these structures affect the impedance of the feedline/RF antenna, and the level of isolation these structures provide.
If an EM field solver is used, near-field and far-field simulations can be used to determine the areas of the PCB layout where strong emissions occur. Once these areas are identified, along with the frequencies emitted, it will help determine which type of isolation strategy should be used. It is best to use the frequency domain directly (FDFD method) rather than using the Fourier transform to convert the FDTD results.
RF antenna design and layout creation requires extra attention to detail, so it makes sense to take extra care to ensure isolation and signal integrity of the RF design.
